Shear wave elastography has been known for several years as an efficient technique for detecting an inhomogeneity of elasticity in a soft solid, such as a tumour. This technique is based on the detection of shear waves propagation speeds. Such a detection can be based on ultrasonic technology or on a magnetic resonance imaging (MRI) technology.
Shear waves can also be used to increase locally the temperature of a soft solid. This can be used as a basis for a thermal mapping method or for a method for thermally treating a target region of a soft solid, including non therapeutical aspects in the case of a non living soft solid, such as cheese or a part of a prosthesis.
In a soft solid, shear waves propagate at a speed in the range of 1 to several meters per second (m/s) and this speed can be used to characterize a target region of a soft solid, since the speed pattern of these waves allows generating images representative of the shear elastic modulus of this target region. This shear elastic modulus approximatively corresponds to the elasticity which can be sent by palpation and is ranging from a few hundreds Pa to a few thousands kPa.
In the field of therapeutical physics, an article published in Physical Review Letters (PRL 100, 064 301 (2008)) entitled «Time reversal of elastic waves in soft solids» by Catheline et al. teaches that a shear wave generated by a single generator and propagating within an agar-gelatin phantom can undergo a time reversal. Agar-gelatin can be used from a theoretical point of view, but such a material is not representative of soft solids that could be used for practical and industrial applications. This derives from the damping feature of the real soft solids with respect to shear waves. This damping feature makes this known method relatively inaccurate.
Moreover, this known method is based on a linear behaviour of the agar-gelatin phantom which constitutes the medium through which the shear waves propagate. In some cases, it might be necessary to obtain a non-linear behaviour of the medium, which cannot be obtained with the known methods because the induced stress level is not high enough for this. Actually, because of their intrinsic technology, none of the commercial elastography methods is able to deliver a shear wave amplitude in the non linear regime.
In fact, the linear behaviour of a medium is a simplistic approximation of the actual behaviour of a soft solid which might not be sufficient to distinguish two different portions of a target zone. For instance, a benign tumour and a malignant tumour might have the same elastic behaviour, which does not allow a differentiation of these tumours with a shear wave imaging method based on an elastic behaviour of the soft solid. On the other hand, a benign tumour and a malignant tumour might have different non-linear behaviours, which allows differentiating them under some circumstances.
Similar limitations occur when one needs to significantly increase the temperature of a target region of soft solid. If the stress level obtained with the shear waves is not high enough, the temperature increase of the medium is not significant.